You’re right, which is why it should be slowly and continuously changed along with the times and through the mechanisms in place to do so, not drastically and sweepingly every 50 years or so. You don’t throw out and replace the constitution or other foundations of law without massive societal upheaval, which is just as bad as permanent stagnation.

The United States is a republic of States. In it all states are equal in the union. That’s why the senate is locked at two senators per state and why all states have equal say on amendments.

And all that is working as intended, because the US is a gestalt entity created of 50 states cooperating in concert with each other. We have an electoral college to balance the power between the biggest and smallest states so that every state gets representation under a representational republic. Extrapolate out to other similar organizations like the European Union. Would Luxembourg join or remain in the EU if votes were done by direct democracy of the population of each member country and Germany alone nearly outnumbers the bottom half of the EU by population? Each member state deserves representation. The same is true for the US, where it’s illegal to leave the union after you’ve joined.

The House of Representatives are the voice of the actual people (unfairly restricted in size which needs to be fixed but that’s another story), the Senate is supposed to be the voice of the States (but we made them directly elected instead of appointed by the state governments making them just a super version of the House but that is yet another conversation), and the President to run the government and act as our figurehead. This is supposed to allow the vastly different needs of each state and for the differing needs between urban and rural areas to be represented. This is why yes, if your amendment cannot convince 75% of states that it’s a good amendment, it probably shouldn’t pass in the first place.

TL/DR: Direct democracy is practiced at the local and state level, then representationally at the federal level because we are a republic of cooperating states that each have their own needs and desires. We have fucked with/fucked up how that representation works in the republic for better or worse, and the system as designed allows the possibility for a tyranny of the minority because it was the only way to prevent permanent tyranny of the majority.